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Homework Help: Reynold number

  1. Feb 15, 2017 #1
    1. The problem statement, all variables and given/known data

    In this link , i was told that Reynold number = ρvD / μ ,

    However in the notes below , i was told that Reynold number = ρvR / μ .

    2. Relevant equations

    3. The attempt at a solution
    I think Reynold number = ρvR / μ is wrong , because i learnt that Reynold number = ρvD / μ in fluid mechanics , however , in the hydraulics notes , i was told that that Reynold number = ρvR / μ . i'm very confused now

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  3. Feb 15, 2017 #2


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    I agree. The characteristic length is the hydraulic diameter and it is confusing to use the term "hydraulic radius".
  4. Feb 15, 2017 #3
    It doesn't really matter, as long as the meaning is established within the context of the system being analyzed. Anyway, it's Reynolds number, not Reynold number.
  5. Feb 15, 2017 #4
    So , which is correct ? ρvD / μ or ρvR / μ ??
  6. Feb 15, 2017 #5
    So , do you mean ρvD / μ is correct ? and ρvR / μ is wrong ?
  7. Feb 15, 2017 #6
    Both are correct, depending on how it is used.
  8. Feb 15, 2017 #7
    Can you explain further when we need to use ρvD / μ or ρvR / μ ???
  9. Feb 15, 2017 #8
    In a given problem, you just need to specify which definition is being used. For example, the laminar-turbulent transition occurs at ##\rho v D/\mu=2100## for flow in a pipe, but it also occurs at ##\rho v R/\mu=1050##. So you have to tell which definition of the Re you are using.
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